Modern industrial, commercial, aerospace and military systems depend critically on reliable pumps for fluid handling. The trends in fluid handling systems are toward smaller, more distributed and more portable systems for increasing uses in instrumentation and control.
Although important advances in pump technology have been made in the past few decades, progress has reached saturation in terms of ability to reduce pump size, weight and power requirements. There is a significant gap between the technology for conventional pumps, including the so-called "micropumps," and MEMS pumps that are based on silicon micromachining and microelectronics technology.
The pumping capability of MEMS pumps is placed in the microliters to tens of milliliters per minute range. This makes them useful for applications such as implantable systems for drug delivery or micro dosage in chemical analysis systems but such pumping speeds are many orders of magnitude smaller than those required in sampling applications.
Conventional pumps that are commercially available have capacities that range from tenths of a liter per minute to several hundreds of liters per minute. Most of these pumps require large amounts of power. Even the smaller pumps are typically in the size range of 10-50 cubic inches. There are also commercially available micropumps that operate with lower input power, but have capacities below one liter per minute.
A number of United States patents have been granted on apparatus and devices generally relating to microvalve construction and control. For example, U.S. Pat. No. 5,082,242 to Bonne et al describes a microvalve that is an integral structure made on one piece of silicon such that the device is a flow through valve with inlet and outlet on opposite sides of the silicon wafer. The valves are closed by contact with a valve seat where surfaces must be matched in order to avoid degradation of valve performance. Two patents, U.S. Pat. Nos. 5,180,623 and 5,244,527 are divisional patents relating to the first mentioned patent.
Another family of patents describe fluid control employing microminiature valves, sensors and other components using a main passage between one inlet and exit port and additionally a servo passage between inlet and outlet ports. The servo passage is controlled by a control flow tube such that tabs are moved electrostatically. U.S. Pat. No. 5,176,358 to Bonne et al teaches such a fluid regulating device, while divisional U.S. Pat. Nos. 5,323,999 and 5,441,597 relate to alternative embodiments.
An additional concept is disclosed by Wagner et al in the June, 1996, edition of the IEEE Journal, pages 384-388, in which two buckled Si/SiO.sub.2 membranes spanning air filled cavities having enclosed driving electrodes. A coupled membrane system is disclosed in which a first silicon membrane is switched by electrostatic force which, in turn, presses air through a channel to push the second silicon membrane up.
In both of these patented systems and in the concept described by Wagner et al, silicon semiconductor chips are employed. Silicon technology is, in fact, a host for a number of microsensors. The possibility of fabricating fully integrated systems led to the development of some of the above described valves and the like. However, the displacements available at the microscale and the materials available in silicon technology are not the best for such applications. The achievable pumping rates are very small (.mu.l to ml/min) at the best. Additionally the structures tent to become complicated and expensive. Of major concern also is the fact that silicon is not compatible with many biological materials, thus eliminating virtually an entire field of end use.
Current sampling pumps for vapor and particle detection are much larger than the instruments they support. In order to be effective for many missions, the sampling rate should be comparable to human breathing, i.e., 10 liters per minute (1 pm) or more. The pumps must supply this flow against pressure drops of one psi or more, corresponding to pneumatic output loads exceeding a watt and input power requirements exceeding ten watts. Current system using rotating motors are power hungry, noisy and have limited lifetimes. Mesoscopic pumps with no rotating or sliding parts and high electrical-to-pneumatic conversion efficiencies would be able to dramatically increase the capabilities and effectiveness of military systems that detect chemical, biological, explosive and other agents.
Use of silicon as a component for these systems has proven difficult, particularly in three areas. First, micromachining the desired curved surface in silicon is a problem; second the choice of materials is severely limited; and third, achieving the dimensions required for high pumping rates is almost impossible. Fabrication constraints result in a reduced radius of curvature at the supports, reduced travel of any diaphragm, and unidirectional actuation, all of which contribute to reduced pumping efficiency.
It would be a great advance in the art if a mesopump could be developed that would be able to supply pumping speeds and maximum pressures similar to conventional pressures at dimensions and power levels that are an order of magnitude smaller.
Another advantage would be if a mesopump would be available that used materials that are compatible with most, if not all, materials likely to be processed.
A specific advantage would be if a mesopump would be devised in which only one electrode per elementary cell is needed to move the diaphragm to operate the pumping function.
Other advantages will appear hereinafter.